Conventionally, this sort of vehicle air conditioning apparatus includes: a compressor driven by an engine as a power source of a vehicle; a radiator provided outside the vehicle interior; and a heat exchanger provided inside the vehicle interior. With this vehicle air conditioning apparatus, a cooling operation is performed by: releasing the heat from the refrigerant discharged from the compressor in the radiator; absorbing the heat into the refrigerant in the heat exchanger; and supplying the air subjected to a heat exchange with the refrigerant in the heat exchanger to the vehicle interior. In addition, such a conventional vehicle air conditioning apparatus includes a heater core and perform a heating operation by: releasing the exhaust heat from the cooling water used to cool the engine in the heater core; and blowing the air subjected to a heat exchange with the cooling water in the heater core to the vehicle interior. Moreover, such a conventional vehicle air conditioning apparatus performs a heating and dehumidifying operation by: cooling the air to be supplied to the vehicle interior to a required absolute temperature in the heat exchanger for dehumidification; heating the cooled and dehumidified air in the heat exchanger to a desired temperature in the heater core; and blowing the heated air to the vehicle interior.
The above-mentioned vehicle air conditioning apparatus uses the exhaust heat from the engine as a heat source to heat the air for a heating operation, or a heating and dehumidifying operation. Generally, an electric car uses an electric motor as a power source, and it is difficult to acquire the exhaust heat that can heat the air to be supplied to the vehicle interior by using the electric motor without an engine. Therefore, the above-mentioned vehicle air conditioning apparatus is not applicable to electric cars.
To address this issue, a vehicle air conditioning apparatus has been known in the art, as applicable to electric cars. The vehicle air conditioning apparatus includes a refrigerant circuit having an electric compressor, an indoor heat exchanger and an outdoor heat exchanger; and an electric heater, and perform a heating operation to heat the vehicle interior by using either or both the heat radiated from the indoor heat exchanger by driving the compressor and the heat radiated from the heater (see, for example, Patent Literature 1).
Also, another vehicle air conditioning apparatus has been known in the art, as applicable to electric cars. The vehicle air conditioning apparatus includes: an electric compressor; a heat medium heating radiator that releases the heat from refrigerant to heat the heat medium; an air cooling heat exchanger that absorbs the heat into the refrigerant to cool the air blowing to the vehicle interior side; an outdoor heat exchanger that is provided outside the vehicle interior and that performs a heat exchange between the outdoor air and the refrigerant to release the heat from the refrigerant or absorb the heat into the refrigerant; a heat medium circuit that allows the heat medium heated in the heat medium heating radiator to flow through; an air heating radiator that releases the heat from the heat medium flowing through the heat medium circuit to heat the air blowing to the vehicle interior; and a heat medium heater that heats the heat medium flowing through the heat medium circuit by electric power (see, for example, Patent Literature 2). This vehicle air conditioning apparatus performs a heating operation by: releasing the heat from the refrigerant discharged from the compressor in the heat medium heating radiator; and absorbing the heat into the refrigerant after the heat release in the outdoor heat exchanger. Moreover, this vehicle air conditioning apparatus performs a heating and dehumidifying operation by: releasing the heat from the refrigerant discharged from the compressor in the heat medium heating radiator; and absorbing the heat into the refrigerant after the heat release in the air cooling heat exchanger and the outdoor heat exchanger. With this vehicle air conditioning apparatus, the heat medium heater can heat the heat medium circulating in the heat medium circuit.
With the vehicle air conditioning apparatus, if the heating operation is performed while the outdoor air temperature is low, the evaporating temperature of the refrigerant drops in the outdoor heat exchanger, so that a frost is likely to be formed on the outdoor heat exchanger. If a frost is formed on the outdoor heat exchanger, the outdoor heat exchanger cannot acquire required quantity of heat. This causes a decrease in the performance of the heating operation.
To address this issue, a vehicle air conditioning apparatus has been known in the art, which performs a defrost operation to remove a frost formed on the outdoor heat exchanger (see, for example, Patent Literature 1).